Arthropod-borne viruses (arbovirus) are responsible for a significant disease burden worldwide. Here, we will investigate the mosquito-virus-host interaction - an important area of research for all mosquito- borne viruses. Mosquito saliva potentiates viral replication for several different arboviruses, depending on the mosquito species, virus and experimental system; however, the mechanism(s) of enhanced viral replication is not well understood. Investigating these mechanism(s) and identifying the responsible salivary factors are the objectives of this proposal. Our model for these studies will be West Nile virus (WNV), one of its enzootic vectors, Culex tarsalis, and mice as the vertebrate host. Our previous studies showed that mosquito saliva enhances early viral replication in this model. We will build on these studies by pursuing three related aims: 1) Determine how mosquito saliva affects viral production. Since greater viral production is due to either greater numbers of cells infected and/or greater yield per cell, we will explore both possibilities in thes studies. 2) Use a systems biology approach to examine the host response to WNV in the presence of mosquito saliva. Since the enhanced viral replication occurs early after infection, we hypothesize that the innate immune response is inhibited by mosquito saliva. We will use microarray studies to examine differences in expression profiles in target tissues during WNV infection with and without mosquito saliva. 3) Identify the mosquito salivary component(s) responsible for enhancement of WNV infection. Our preliminary studies have narrowed the possible candidates to two protein families found in Culex tarsalis and Aedes aegypti, but not in Anopheles gambiae. We will determine which of these candidates is responsible for viral enhancement and subsequently test it as a possible vaccine. Upon the completion of these aims, we will have made significant contributions to understanding arbovirus pathogenesis in the context of mosquito transmission and laid the groundwork for the development of novel targets to reduce viral transmission.